Exploring watershed effects on nutrient concentrations in shallow lakes through stable isotope analysis

Biogeochemistry patterns in shallow lakes are influenced by both in-lake factors such as ecosystem state as well as watershed-level factors such as land use, but the relative importance of in-lake versus watershed factors is poorly known. This knowledge gap makes it difficult for lake mangers to prioritize efforts on watershed versus in-lake strategies for stabilizing the clear-water state. We studied 48 shallow lakes in Minnesota, USA to assess the relative influence of lake size, land use in watersheds, and ecosystem state (turbid versus clear) on water column total nitrogen (TN) and total phosphorus (TP), as well as delta N-15 and delta C-13 in three species of fish. Our land use categories included natural areas, row crop agriculture, and all agriculture (row crops plus alfalfa). A model selection approach revealed different control mechanisms on the behavior of stable isotopes and nutrients. delta C-13 ratios in fish were most strongly influenced by lake size, while delta N-15 ratios were influenced by all agriculture in watersheds. In contrast, water column TN and TP concentrations were influenced by the in-lake factor of ecosystem state, with both nutrients lower in the clear state. We detected no effects of land use on TN or TP concentrations, likely due to strong effects of ecosystem state masking watershed effects. However, the strong relationship between agriculture and d15N in fish indicated that watersheds did influence nutrient processing in shallow lakes, and that effects are not a legacy from past watershed events. Collectively, these observations indicate that lake managers should minimize agricultural intensity in shallow lake watersheds to facilitate the clear-water state, which will, in turn reduce water-column TN and TP relative to the turbid state.

Automated summary

Biogeochemistry patterns in shallow lakes are influenced by both in-lake factors and watershed-level factors. The relative importance of these factors is not well understood. A study in Minnesota, USA demonstrated that lake size had the strongest influence on fish δC-13 ratios, while all agriculture in watersheds influenced fish δN-15 ratios. Water column TN and TP concentrations were influenced by the in-lake factor of ecosystem state, with lower concentrations in the clear state. Land use did not have a significant effect on TN or TP concentrations, likely due to the masking effect of ecosystem state.